In recent years, collapses have been occurring frequently in ferroconcrete viaducts and tunnels of the Shinkansen Line and have become a serious social problem. Leaving aside the issue of causes of such collapses, ferroconcrete structures that had been built in rapidly increasing numbers in 1960-1970's that was a period of high growth of Japanese economy have been aging under the effect of exhaust gases of vehicles and acid rains and also because of the increased volume of traffic. Ferroconcrete structures are expected to have a service life of no less than 50 years, but most of the structures built in the period of high growth of Japanese economy are approaching the life limit because of worsening of the environment surrounding the ferroconcrete structures. However, service life of ferroconcrete structures can be greatly extended by conducting appropriate maintenance and repair within appropriate periods.
For example, in bridges, such as steel bridges, which have ferroconcrete deck slabs, concrete deck slabs are connected to each other in the bridge pier portions via an expansion unit. As shown in FIG. 20, an expansion unit 202 is inserted between the end portions of concrete deck slabs 200, 201. Steel members 202A, 202B of the expansion unit 202 are integrated with the end portions of respective concrete deck slabs 200, 201, and both steel members 202A, 202B are set so that comb-like pawls are mutually engaged. The expansion unit 202 provided at the end portions of concrete deck slabs 200, 201 represents a zone where the largest damage occurs and the replacement of damaged units is required.
Replacing the expansion unit 202 is an operation comprising the steps of cutting the end portions of concrete parts 203 of concrete deck slabs 200, 201 together with the internal reinforcements 204 with a rotary cutter or wire sawing apparatus, then removing the cut ends of concrete deck slabs 200, 201 and steel members 202A, 202B, breaking the cut surface of the concrete parts 203 to a thickness of about 10-15 cm with a breaking machine over the entire surface, thereby exposing the end portions of reinforcements 204, then installing a new steel member 202A, extending the exposed portion of reinforcement 204 by flare welding a stud reinforcement 205 with an about 10-cm overlapping, arranging the other reinforcement, and finally placing concrete.
The most troublesome of those operations is breaking the concrete. To minimize traffic restriction, replacing the expansion unit 202 of this type is usually conducted as an intensive operation. For this reason, the breaking process is often extended far into the night and the generated noise greatly annoys people living nearby. Furthermore, in case of extreme noise, the operation itself is sometimes interrupted because of complaints from people living nearby or by use of force.
On the other hand, there are zones where chronic traffic jams occur due to the increased volume of traffic, the clear examples being the Tokyo-Nagoya Expressway, Nagoya-Kobe Expressway, and Tokyo Expressway, and widening of traffic lanes has been conducted in various regions to relieve such traffic jams. The operation of widening a traffic lane will be briefly explained based on FIG. 21, this operation having features identical to those of the above-described operation of replacing an expansion unit. When the end portion of an existing concrete deck slab 300 shown in FIG. 21(a) is extended, first, a concrete part 301 is cut (see FIG. 21(c)) together with an internal reinforcement 302 along a cutting line L1 shown in FIG. 21(b). Then, the concrete part 301 is broken with a breaking machine or water jet till a breaking line L2 (see FIG. 21(d)) at a distance of 10-15 cm from the cutting surface 303 and the distal end portion of reinforcement 302 is exposed (see FIG. 21(e)). A stud reinforcement 304 is then joined by enclose welding or mechanical joining to the end portion of existing reinforcement 302, with an overlap of about 10 cm, and finally concrete 305 is placed (see FIG. 21(f)). The reference symbol 306 in the figure indicates a reinforcement overlapping allowance. In this case, the operation of breaking the concrete part to a thickness (a distance from the cutting surface to a cutting line L2) shown in FIG. 21(d) also was the major problem in terms of extending the work time and generating noise.
Further, in the operation of repairing and modifying the inner surface of a tunnel, a new reinforcement is installed along the inner surface of the existing tunnel and the aforesaid flare welding or enclose welding or mechanical joining has been used as means for joining the reinforcements. The operation of joining the reinforcements in this case required much time and labor, making it difficult to reduce cost.
The aforesaid enclose welding represents a technology for coaxial welding of reinforcement of the same diameter. However, this welding operation takes much time and has poor operation efficiency making it unsuitable for commercial applications. Furthermore, because of a high thermal capacity of generated heat, material properties of the reinforcement are changed. In addition, a concrete breaking operation is basically required to guarantee a welding allowance of prescribed length. Further, stud welding technology has generally been employed for welding a reinforcement to the surface of a steel plate, but stud welding has been conventionally recommended for welding to iron plates with a surface area of no less 15 cm square and usage thereof in such cases was based on common sense because of a magnetic blow effect. Furthermore, lap joining has also been widely used as technology for joining reinforcements to each other. In the lap joining process, the reinforcements are laid one on top of the other over a length of no less than 30 times the thickness of the reinforcements and tied up with a binding wire. However, welding a reinforcement with a diameter of 20 mm requires a lapping allowance of about 80 cm and is unsuitable for replacement of expansion unites or lateral width expansion of deck slabs.
Thus, it was a matter of common sense that a mating surface for welding a stud reinforcement was a plate with a rather wide area. For this reason, the operations of welding the reinforcements of about the same diameter to each other and welding the reinforcements in an almost horizontal state thereof were absolutely unimaginable within a framework of the conventional stud welding technology. Moreover, the operation of extending an existing reinforcement by welding a stud reinforcement thereto, primarily the operation of replacing an expansion unit, is conducted in a very narrow space, but the conventional stud guns have a structure in which a stud is mounted coaxially, so as to obtain an extension, to a distal end of a drive shaft which is driven linearly so as to protrude forward from a gun body. As a result, a minimum space is required of a size obtained by adding the stud length to the length to the distal end of the drive shaft of the stud gun, and setting itself becomes especially difficult when a long shaped reinforcement is used.
With the foregoing in view, it is a first object of the present invention to provide a reinforcement butt stud welding method suitable for welding of reinforcements of approximately the same diameter and also suitable for good welding of reinforcements in an almost horizontal state thereof.
It is a second object of the present invention to provide a stud welding apparatus suitable for coaxial butt welding of a stud reinforcement to an end portion of a horizontally laid existing reinforcement in a narrow work space.
It is a third object of the present invention to provide a repairing/expanding method for a ferroconcrete structure by which in the method comprising the steps of partially cutting the existing ferroconcrete structure and constructing a new ferroconcrete portion by using the above-mentioned stud welding apparatus, the operation of breaking the concrete portion can be reduced to a minimum, thereby shortening the term of works and reducing noise generation, this method being especially suitable for replacing an expansion unit disposed in a joint portion of deck slabs of a bridge, lateral width expanding the deck slabs of a bridge, or reinforcing and modifying the internal surface of a tunnel.